Joint replacements may be generally divided into two designs—total arthroplasty and hemiarthroplasty. Total arthroplasty designs generally replace both sides of the joint, for example, a total hip replacement is made up of an acetabular cup which articulates with a femoral component comprising a ball and stem. A hemiarthroplasty generally only replaces one side of the joint. Using the hip again as an example, a hip hemiarthroplasty uses the native acetabular cup to articulate with a prosthetic ball and stem. Other examples include the shoulder where a total arthroplasty replaces both the humeral and scapular articular surfaces, while a hemiarthroplasty only replaces the articular surface of the humerus.
In some joints, with more complex biomechanics than the two ball and socket type joints mentioned above, both total and hemiarthroplasty designs have been used, with mixed success. An example of a joint with complex biomechanics is the first carpometacarpal joint of the thumb. This joint is made up of the trapezium bone, and the first metacarpal bone. During movements of this joint, the metacarpal bone moves in the following ways in relation to the trapezium; flexion and extension, abduction and adduction, internal rotation and external rotation. The metacarpal bone also translates across the trapezium bone. Movements of the thumb digit are enabled by a combination of any or all of these motions in different ratios depending on what motion is taking place.
As the movement of the thumb digit is determined by a combination of muscles activations, and in turn a combination of bony movements as described above, it has been found that the axis of rotation of the thumb is not always in the same place. The axis of rotation of the thumb joint moves depending on what movement of the thumb is taking place. The axis of rotation during abduction-adduction movements is in the base of the metacarpal, while the axis of rotation during flexion-extension is in the trapezium. In true terms, the axis of rotation of the joint shifts between these two points in line with the ratio of bone movements taking place.
While the saddle-shaped geometry of the CMC joint is largely responsible for the wide range-of-motion and functionality of the joint, the corresponding complex biomechanics are thought to be one of the primary causes of the high failure rate of both total arthroplasty and hemiarthroplasty implants which have been designed for this joint. This highly mobile joint may also predispose it to instability and osteoarthritis.
The other significant cause theorised as to the high failure rates of implants is the significant forces transmitted through the joint during forceful motions of the thumb, such as pinching, grasping or twisting. It has been shown that the forces transmitted through the CMC joint are up to ten times that exerted on the tip of the digit.
As all current total arthroplasty implants for the CMC joint require the implantation of one part in the trapezium, a common failure mechanism for this type of design is subsidence or failure of the trapezium element such as the cup and socket. By placing the socket in the trapezium, the point of rotation for all movements is now limited to one point, while it is known that the axis of rotation moves between the trapezium and the metacarpal in the native joint. The trapezium must also be surgically resected to allow placement of the cup and socket, decreasing the viable bone stock available. Therefore, by limiting the point of rotation to one position outside the natural shifting axis of rotation, placing this point in poor quality limited bone, and then subjecting this point of stress to multiplied forces of significant amounts, it is unsurprising that failure of the ball and socket element of total arthroplasty is a common failure mechanism.
Hemi-arthroplasty designs have been developed in an effort to avoid having to place the point of rotation in the trapezium, and instead, modify only the articular surface of the metacarpal. These designs have had limited success clinically, with no statistically significant difference in implant survival over total arthroplasties (Kurkhaug Y, Lie S A, Havelin L I, Hove L M, Hallan G. The results of 479 carpometacarpal joint replacements reported in the Norwegian Arthroplasty Register. Journal of Hand Surgery (E) 2014 39 (8): 819-825). As these hemiarthroplasty implants are uniblock i.e. one part designs, the forces applied to the implant from the trapezium as it moves tend to be transmitted to the stem of the implant. This has caused stem loosening and implant failures (Naidu S H, Kulkami N, Saunders M, Titanium Basal Joint Arthroplasty: A Finite Element Analysis and Clinical Study, The Journal of Hand Surgery. 2006 31(5) 760-765). A uniblock hemi-arthroplasty device is described in U.S. Pat. No. 8,303,664.
Hemi-arthroplasty designs generally involve the modification or remodelling of the trapezium bone into a specific shape to accommodate the implant. This compromises the integrity of the trapezium bone. Another failure mode of hemiarthroplasty designs is luxation, i.e. dislocation of the implant from the surgically remodeled trapezium (Pritchett J W, Habryl L S. A Promising Thumb Basal Joint Hemiarthroplasty for Treatment of Trapeziometacarpal Osteoarthritis. Clinical Orthopaedics and Related Research. 2012; 470(10):2756-2763; Martinez de Aragon J S. Early Outcomes of Pyrolytic Carbon Hemiarthroplasty for the Treatment of Trapezial-Metacarpal Arthritis Journal of Hand Surgery, Volume 34, Issue 2, 205-212).
Patents for two-part hemiarthroplasty devices have been noted, however, these designs require the dynamic reconfiguration of the two parts relative to each other to achieve an alteration in the point of motion. FR2912051 discloses such a device. While this device provides two articulation points, the two parts of the device need to be separated (dynamic reconfiguration) to achieve a movement of the axis of rotation (as shown in FIG. 4 of FR2912051). As any device implanted in this joint would be expected to be under physiological compressive forces of significant amounts, this dynamic reconfiguration would be impossible to achieve to a degree that would provide any meaningful biomechanical or clinical impact.
It is an object of the invention to overcome at least one of the above-referenced problems.